Pulmonary embolism

A pulmonary embolism often starts when a blood clot forms within a vein deep inside the body -- in a leg or a hip, for example, and then detaches from the vein's wall and moves through the bloodstream into the heart.

There the heart may pump the clot into the pulmonary artery leading to the lungs, where it can lodge and prevent the lungs from receiving fresh blood. If that happens, the blood supply is choked off, starving cells of oxygen. The cells then die.

Pulmonary clots that block the main artery feeding blood to the lungs strike an estimated 500,000 adults a year in the United States and cause about 50,000 deaths.

According to the American Medical Association, "people who are confined to bed, especially after pelvic surgery or surgery involving the hip or knee, are at increased risk."